Metallic or ceramic moldings can be produced by injection molding, extrusion or compression molding of thermoplastic compositions which, in addition to metal powders and/or ceramic powders, comprise an organic binder. These are organic polymer molding compositions with a high filler level. After the thermoplastic composition has been shaped to give a green part, the organic binder is removed and the resulting debindered green part (=brown part) is sintered.
The first binders for the powder injection molding process were based generally on mixtures of polyethylene or polypropylene and wax. In this case, the green body is first freed of the wax by melting and the residual binder is burnt out by slow pyrolysis. For the melting, the green parts have to be introduced into a powder bed because there is virtually no green strength as a result of the melting. Later binder systems for thermal debindering do not involve melting because the costly and inconvenient embedding of the green parts into powder and subsequent excavation are much too time-consuming.
Typically, an improved binder system for the complete thermal debindering consists of several components, as described, for example, in DE 1992 5197. These components are released gradually from the shaped bodies on heating at different temperatures, such that the typically relatively low binder constituent is still present up to at least 400° C. and can be considered to be residual binder. Purely thermal debindering takes 1 to 3 days and is thus extremely slow.
A still further-improved process is that of solvent debindering, in which binder systems which comprise binder components of different solubility are used. For debindering of the green part, one binder component is first removed by solvent extraction, after which the remaining residual binder component, which is insoluble or very sparingly soluble in the solvent, is in turn removed from the molding by a slow thermal decomposition (e.g. U.S. Pat. No. 4,197,118 or EP 501 602). In the course of this, the melting range of the residual binder is passed through and plastic deformation of the powder molding is unavoidable as a result.
WO 2011/016718 A1 describes a powder injection molding process in which a binder mixture of a polymer, for example POM, and a nonpolymeric solvent (molar mass<300 g/mol, melting point>RT) is used for the polymer. The aforementioned solvent is first leached out or else evaporated. The remaining polymer is removed by thermal debindering. A disadvantage in this process is that such binders, when mixed with powder and when processed in an injection molding machine, already vaporize the nonpolymeric solvent. The low molecular weight component is sweated out at the green part surface and soils the injection mold. Furthermore, the green part strength is distinctly reduced.
R. M. German writes, in his handbook “Powder Injection Molding”, MPIF 1990, Chapter 4, page 115, about solvent debindering:
“The two constituents in the binder are often present in roughly equal proportions. This allows each to remain interconnected throughout the pore structure between the particles. The binder interconnectivity can be easily maintained with as little as 30 volume percent of either constituent. Successful binder formulations therefore contain 70 to 30% by volume of a major component.”
A further process for debindering the green part according to the prior art is based on catalytic debindering by treatment of the green part in a gaseous acid-containing atmosphere at elevated temperature. EP-A 0 413 231 discloses, for example, a process for producing an inorganic sintered molding, in which a mixture of a sinterable inorganic powder and polyoxymethylene as a binder is shaped to a green body, and the binder is then removed by treating the green body in a gaseous, acid-containing, for example boron trifluoride- or HNO3-comprising, atmosphere. Subsequently, the green body thus treated is sintered. Examples of sinterable powders are both oxidic ceramic powders such as Al2O3, ZrO2, Y2O3, and nonoxidic ceramic powders such as SiC, Si3N4 and metal powders.
With a binder phase consisting exclusively of POM, however, satisfactory results are not obtained in practice since the sinter densities are too low.
EP-A 0 444 475 describes binder compositions which are suitable for ceramic shaped bodies and comprise, as well as polyoxymethylene, poly-1,3-dioxolane, poly-1,3-dioxane or poly-1,3-dioxepane as an additional soluble polymer, or aliphatic polyurethanes, aliphatic polyepoxides, poly(C2-C6-alkylene oxides), aliphatic polyamides or polyacrylates, or mixtures thereof, as a polymer dispersible in POM.
EP 0 465 940 A1 and DE 100 19 447 A1 describe thermoplastic molding compositions for the production of metallic shaped bodies, which comprise, in addition to a sinterable pulverulent metal or a pulverulent metal alloy, a mixture of polyoxymethylene homo- or copolymers and a polymer immiscible therewith as a binder. Useful additional polymers include polyolefins, especially polyethylene and polypropylene, and also polymers of methacrylic esters such as PMMA (EP 0 465 940 A1). DE 100 19 447 A1 describes binders for inorganic material powders for production of metallic and ceramic shaped bodies, these binders comprising a mixture of polyoxymethylene homo- or copolymers and a polymer system composed of polytetrahydrofuran and at least one polymer formed from C2-8-olefins, vinylaromatic monomers, vinyl esters of aliphatic C1-8-carboxylic acids, vinyl C1-8-alkyl ethers or C1-12-alkyl(meth)acrylates.
WO 2008/006776 A1 describes binders for inorganic material powders for production of metallic shaped bodies, these binders being a mixture of polyoxymethylene homo- or copolymers and a polymer system formed from C2-8-olefins and poly-1,3-dioxepane or poly-1,3-dioxolane.
When the aforementioned POM binder systems are used, the green parts are debindered catalytically by treatment of the green part in a gaseous acid-containing atmosphere of, for example, hydrogen halides, formic acid or nitric acid at elevated temperature. This depolymerizes the polyoxymethylene homo- or copolymers without residue, followed by a slow thermal residual debindering of the remaining polymer. Here too, the melting range of the residual binder is passed through and plastic deformation of the powder molding is unavoidable as a result. The residual binder content in the case of catalytic removal is generally about 10%. Owing to the lower residual binder content, plastic deformation is typically less marked than in the case of solvent debindering, where the residual binder content is typically 30 to 70%.
In the case of reactive powder surfaces, catalytic debindering with the aforementioned acids, especially with nitric acid, can cause problems as a result of a reaction of the acid with the surface. This reaction may be so pronounced that the debindering stops as a result of pore blockage after a penetration depth of a few tenths of a millimeter. For example, HNO3 debindering in the case of copper and copper-containing alloys is impossible or, in the case of a low Cu content, possible only to a limited degree because the voluminous nitrates block access to the molding interior. Similar behavior is known from cobalt.
However, experience has shown that problems do also occur in the case of other metals where no reaction with HNO3 is obvious and the debindering proceeds completely normally. In these cases, possibly only a surface reaction takes place and the debindering rate is adversely affected to a barely noticeable degree, if at all. Nevertheless, in the sintered product, an increase in the oxygen content is found, for example in the case of titanium, or a loss of carbon in the case of carbon-containing alloys, for example of iron, as a result of the reaction to give gaseous carbon oxides.
Further examples of powders in which there is no directly obvious occurrence of reactions with HNO3 are W, V, Mg, Mn, and ceramic powders such as AlN and Si3N4. Especially in the case of alloys comprising reactive metals, for example Al- and/or Ti-containing superalloys such as IN713C, MAR 246, GMR 235 and IN 100, such surface reactions are not disruptive for the progress of debindering, but the Al- and Ti-containing oxide layers resulting therefrom are no longer reducible later in the sintering step, and these alloy elements are then unavailable or only partly available for the alloy formation; the material properties of the sintered product are worse or even unusable.
Catalytic debindering with oxalic acid, even for oxidation-sensitive sinter materials such as WC/Co and Cu, is described in WO 94/25205. However, catalytic debindering with oxalic acid in direct comparison with HNO3 is much slower, and the metering of oxalic acid in solid form is problematic, and so there has been no industrial use thereof to date.
All available literature about powder injection molding from the last 30 years thus states that the organic binder must consist of several components; in general, there are at least two components, of which one component is removed in a first debindering step and a second component remains in the molding as residual binder.
The function and importance of this residual binder, called “backbone”, is explained by the fact that the backbone must ensure basic strength in the brown part in order to enable the transport of the brown parts (for example to quality control tests, or from the debindering furnace to the sinter furnace). In addition, the residual binder during the early stage of the sintering operation should guarantee that the moldings are intact, since the diffusion processes which lead at first to contact formation between the powder particles and later to densification typically set in only at approx. 600-1000° C. Below this temperature, a debindered molding without residual binder would correspond to a pure packing of powder particles, effectively a sandcastle, without any strength.
R. M. German writes, in his handbook “Powder Injection Molding”, MPIF 1990, Chapter 4, page 99: “The binder is a temporary vehicle for homogeneously packing the powder into the desired shape and then holding the particles in that shape until the beginning of sintering.”
The content of the residual binder varies from approx. 10 to a maximum of 70% by weight of the binder phase, the content being dependent on the primary debindering method selected and the polymer type.
According to the prior art, the residual binder is removed without exception by thermal decomposition. The temperature at which the residual binder leaves the brown part depends on the polymer selected and on the selection of the furnace protective gas, but is typically within the temperature range of 300 to 600° C., especially 400 to 500° C.
A disadvantage of the customary thermal residual debindering is that this is an original source for unwanted reactive substances. During the thermal residual debindering, polymer chains are typically cracked and split up into shorter chains. In many polymers, carbon forms as a by-product, and this carbon is very finely distributed and reactive. This reactive carbon can in turn be bound by the reactive metals or alloy elements and form further unwanted secondary phases (carbides).
The residual binder is also a considerable disadvantage for the operation of the sinter furnace, the heating rate of which always has to take account of the thermal decomposition of the residual binder, and problems often exist with the control of the carbon content as a result of carbonization of the residual binder.
The sinter furnace accordingly has to fulfill a particular task within one temperature range (300 to 600° C.) where the control of the furnace is difficult; in terms of power release, the sinter furnaces are designed for the high-temperature range above 1200° C. Since sinter furnaces, especially batch sinter furnaces for MIM, are very expensive due to the molybdenum used, it would be an important cost advantage to not have to pay any attention to thermal decomposition. The higher heating rate possible without residual binder could reduce the cycle time by 20 to 40%.
A further disadvantage is that the sinter furnace is affected by decomposition products of the residual binder, which have to be conducted out of the furnace by complex constructions and usually have to be condensed out, which causes considerable maintenance work.